Although the channel estimation is being studied for a long time, the solutions are not applicable for OFDMA. Thus, new algorithms are needed for such emerging techniques with additional constraints.
A typical estimation for OFDM signals is provided in the document US2006/0205437, for wireless communication systems, which uses preamble. In this document, an averaging is made between first channel estimation based on the preamble and second estimation based on pilots, which are predefined carriers modulated with values known by the receivers. Following this, linear interpolation is used to obtain the channel estimates corresponding to the data.
The problem is that the method of US2006/0205437 cannot be used in fast-varying scenarios, which is a scenario where there are fast varying channel conditions. Indeed, the method proposed in US2006/0205437 would allow estimating the diagonal components of an overall channel matrix describing the transmission channel in the frequency domain. However, this method would not allow estimating the non-diagonal components of the matrix, (i.e. inter-carrier interference (ICI)) terms, which are non-zero in fast-varying scenarios. Therefore, the method of US2006/0205437 is not designed for rapidly varying channels.
A different time interpolation between the pilots is presented in US2005/0243791, however, as in US2006/0205437, the goal is only to estimate the diagonal terms of the channel matrix, which corresponds to the channel coefficients on the carriers.
In US2007/0159959, the data channel estimator is considered to be least squares (LS), linear, cubic, or polynomial interpolation, or another data channel estimation scheme to interpolate the data channel values at the data carrier locations.
Another classical method, disclosed in U.S. Pat. No. 6,314,131, proposes also linear interpolation over multi-slots for the improvement of the channel estimates. However, as in US2006/0205437 and US2005/0243791, the estimation is only done on the carrier channel values missing in the given pilot patterns.
The method only considers the channel coefficients over the symbols. In other words, such methods are lacking the estimation of the ICI terms of the channel matrix which are the dominant terms on the performance of a receiver in fast varying scenarios (high mobility scenarios).
Basis Expansion Modeling (BEM) is one of the possible ways to approximate the time-variation of the channel within a certain time window (thus allowing the estimation of the ICI terms) and has recently taken a lot of attention for fast-varying channel estimation. Basically, this method reduces the complexity as the problem is reduced to estimating the basis coefficients. Among the existing BEMs, a particular attention has been given to the polynomial BEM (P-BEM) for relatively low Doppler spreads. Although P-BEM has an attractive performance when the whole band is available for estimation, it has been shown that with a sparse pilot distribution as in current broadband wireless communication systems such as WiMAX and LTE, the P-BEM cannot be used for estimating directly ICI terms.
The improvement provided by using first order polynomial approximation in P-BEM over two successive OFDM symbols to estimate the channel variation based on given pilot distributions has been investigated. In this approach, the obtained initial estimates are used to compute the modeling parameters in the frequency domain. A similar approach but in a time domain version has also been proposed for two successive OFDM symbols and generalized to multiple OFDM symbols with a polynomial modeling of degree dependent on the number of successive OFDM symbols used in the estimation.
All these approaches have shown interesting results but they are not applicable for OFDM based systems using practical pilot distributions. Indeed, these methods are not suitable for future wireless communications systems since pilots are not defined to be present in successive OFDM symbols.
It is also worth noting that the extension of the already existing methods is not obvious for OFDMA based systems. In particular, in future wireless communication systems, available resources are allocated to users both in time and frequency domains based on standard specific resource units. These resource units occupy a small part of the available bandwidth in a certain time period (a subset of the carriers of a symbol, for a series (e.g. 6 or 7) of symbols), which prevents the usage of the existing schemes for these systems. Moreover, existing methods estimate the channel in the time domain before transferring it into the frequency one, thus requiring additional FFT (Fast Fourrier Transform) processing. The Fast Fourrier Transform is a classical operation for the conversion of a signal received in the time domain to the frequency domain.
Thus, there is a need for defining a method for estimating the ICI terms of the channel matrix, and it should be a method, which is adapted to OFDMA based systems (where resources are allocated to users in the time and the frequency domain) and is usable in fast varying conditions.